In numerous technical applications, there is a need for coupling electrical signals to and from high-speed electronic devices. One particular application is the coupling of electrical signals to semiconductor laser diodes driven by high frequency signals or very short pulses. These devices have low impedance and in order to reduce reflection problems, an impedance matching to e.g. a 50Ω external cable has to be provided. High-speed photodiodes present a similar problem. In order to improve the efficiency and temporal response performance it is necessary to match the relatively high impedance of the photodiode with a low external load e.g. by use of wide-band impedance transformers.
Some solutions for matching different impedance values are present in prior art. In most cases in microwave technology, a narrow band resonant structure is constructed, for instance with stubs of given length. Common to most broadband solutions is that the impedance-matching device tries to create a gradual impedance change between the ends of the impedance matching device. The gradual change is achieved by e.g. varying the transmission line dimensions, the thickness of any dielectric material between the transmission line and grounded parts of the device, the geometry of grounded parts or the dielectric constant of the dielectric material.
However, complex additional requirements or limitations are present. In many recent applications, the device is requested to match impedances typically between 50Ω and 3Ω, and in some cases even from 377Ω down to around 3Ω. Furthermore, if short pulses are used, the impedance matching has to be operable within a large bandwidth. The size of the device is also of crucial interest, since many of the devices connected to it are small. In the case of e.g. laser diodes, the total size should preferably not be larger than about 1-2-cm.
Furthermore, additional effects, such as dispersion, higher order modes and energy loss have to be considered carefully. Finally, such impedance-matching devices also have to be easy and inexpensive to manufacture. The requirements discussed above make the design of well operating impedance-matching devices very difficult indeed. A number of proposals are presented in prior art, each one with pertinent drawbacks.
The problems affecting impedance matching structures known from the prior art can be illustrated with the transmission line transformer (TLT), proposed in U.S. Pat. No. 5,200,719. The structure was designed to match the input resistance of laser diodes to 50 ohms and of photodiodes to low impedances (˜3Ω), allowing considerable improvement of the efficiency and temporal response of the semiconductor devices. The impedance-matching coupling device comprises a dielectric slab of uniform thickness, supporting on the upper face a coplanar transmission line formed by a conducting strip centrally located, alongside which two ground planes are placed. The characteristic impedance of the device undergoes a gradual change of value through a gradual variation of the spacing between lateral and central conductors, as well as through a change of the width of the conductors. The lower surface of the slab supports another conducting ground plane and all ground plane conductors are electrically joined at both ends of the device, as well as on several intermediate points, by shorting straps or wires. By using very high dielectric constant bulk substrates, the size of the TLT can be greatly reduced. However, simulations have shown that the resulting transversal physical dimension requirements limited the transformation impedance level from 50Ω to no less than 8Ω. In this TLT arrangement, the gap to the grounded semiplanes on either side of the line varied from 1.07 mm to 10 μm. Even with this extremely narrow gap, the impedance is not lower than 8Ω at the low impedance side. The fabrication of such an impedance matcher with very small features is very difficult. An additional disadvantage of the TLT described in U.S. Pat. No. 5,200,719 is that it is difficult to obtain substrate materials with low loss at microwave frequencies and very high relative dielectric constant. Yet another disadvantage of the structure is that high dielectric constant bulk substrates introduce large dispersion, which causes problems such as ringing. Furthermore, it has been observed that this structure does not respond above 25 GHz due to the appearance of higher order modes.
Another solution to the problem of matching the impedance of two transmission lines is disclosed in the U.S. Pat. No. 5,119,048. The impedance matching network comprises of two layers of dielectric substrates. A central conductor is disposed between the two layers. Ground planes are located on the surfaces of the substrates that are opposite to the side of the central line and the width of the ground plane metallization along the structure is varied by forming tapered conducting shapes.
One problem with the solution in U.S. Pat. No. 5,119,048 is that there are difficulties in avoiding an air gap between the two dielectric substrates. Therefore, soft substrates are typically used for stripline-like structure in order to facilitate the contacting between the dielectrics. Such soft substrates generally have a relatively low dielectric constant. This in turn leads to impedance-matching devices with a large geometrical extension. This solution also has the drawback of giving rise to large transversal dimensions to match impedances in the range of interest. A typical embodiment according to U.S. Pat. No. 5,119,048 matches impedances of 27 and 50Ω, respectively, in the frequency range between 350 MHz and 1.5 GHz. In many recent applications, this is totally insufficient. The limitation of the useful frequency and impedance range is due to dispersion effects arising in the bulk substrates, low dielectric constant values and size constraints.
In U.S. Pat. No. 5,140,288, another impedance-matching device is disclosed. The device includes a dielectric having a varying thickness between opposing surfaces. The impedance transformation between the two terminals is proportional to the thickness variation of the dielectric.
Besides similar drawbacks as for the earlier discussed solution, this latter device is not very adapted to manufacturing demands. The variation in dielectric thickness is not easy to accomplish for harder dielectric materials. Furthermore, also in this type of devices, severe dispersion exists at higher frequencies. Moreover, at the narrow end of the wedge-formed dielectric part, the lateral extension of the parallel line and ground planes are large compared with the width of the dielectric part, which may induce problems with higher order modes of the created electromagnetic field.
In U.S. Pat. No. 3,419,813, an impedance-matching device is disclosed, which comprises a tapered conductor separated from a ground plane by a dielectric slab. A tapered line section which has an impedance of, for instance, 5 ohms at its low impedance stripline end, requires a greatest width of 7 mm and a total length greater than 5 cm when a PTFE substrate slab of εr=10 and a thickness of 0.635 nm is used. Such dimensions are incompatible with the small dimensions of the packages of the optoelectronic devices.
Therefore, general problems with prior art impedance-matching devices are that the operational bandwidth is limited, high order modes appear at low frequencies, the dispersion causes the device to respond differently at various frequencies, the fabrication is difficult and expensive because of the required tolerance, or the size is too large for accommodation within the package.